The present invention relates to a system for generating high frequency high level noise fields.
High intensity noise fields of specified spectral shapes are required for a number of industrial and technical testing purposes an example of which is the provision of a high level noise field to excite and test aerospace structures in a simulated noise environment of rocket, jet engine and propeller noise. The production of such noise fields in the frequency range above 1.25 KHz has been found to be either impractical or expensive. Certain acoustic test facilities have been using commercially available low or medium frequency noise generators. These generators suffer from high-frequency roll-off above about 500 Hz. A few test facilities have been using aeroacoustic noise sources such as impingement jets, shock cell noise, Hartmann generators or modified Hartmann generators in place of conventional "high" frequency noise generators. The Hartmann generator produces narrow band high level tones and if it is detuned could, less effectively, produce broad band tones. In order to fill up the width of the frequency spectrum with noise, many Hartmann generators would have to be added. However this solution is impractical insofar as the cost of operating multiple Hartmann generators makes the solution uneconomical and further the interaction of the Hartmann signals does not always produce the desired spectrum, as tests have shown.
For a background on the noise testing of articles reference is made to the paper entitled The Use of Hartmann Generators as Sources of High Intensity Sound in a Large Absorption Flow Duct Facility, D. L. Martlew of the National Gas Turbine Establishment at Farnborough, Hants, United Kingdom, (published in the A.I.A.A. March 1975). The paper discusses a large scale noise test facility used in an aeroengine reduction noise program. Another paper of interest is that of D. A. Webster and D. T. Blackstock, Journal of the Acoustic Society of America 63(3), March 1978, pages 687-693 which discusses the interaction of high level high frequency tones with low level broad band noise by collinear propagation.
The problem facing the test facilities is to fill up the produced noise spectrum between 500 Hz and say, 10 KHz in an economical practical fashion.